The present invention relates generally to global navigation satellite systems, and more particularly to improving the positioning quality of global navigation satellite system receivers operating in the differential navigation mode.
Global navigation satellite systems (GNSSs) can determine locations with high accuracy. Currently deployed GNSSs include the United States Global Positioning System (GPS) and the Russian GLONASS. Other GNSSs, such as the European GALILEO system, are under development. GNSSs are used in a wide range of applications, such as surveying, geology, and mapping. GNSS sensors estimating positions and velocities are also integrated into automatic control systems for agricultural and construction machines.
In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the navigation receiver. The satellite signals comprise carrier signals modulated by pseudo-random binary codes. The navigation receiver measures the time delays of the received signals relative to a local reference clock. Code measurements enable the navigation receiver to determine the pseudo-ranges between the navigation receiver and the satellites. The pseudo-ranges differ from the actual ranges (distances) between the navigation receiver and the satellites due to various error sources and due to variations in the time scales of the satellites, and the navigation receiver. If signals are received from a sufficiently large number of satellites, then the measured pseudo-ranges can be processed to determine the code coordinates and time scales at the navigation receiver. A single navigation receiver determining coordinates only from pseudo-ranges operates in a stand-alone mode. For a stand-alone system, the errors in the coordinates are on the order of tens of meters.
To improve the accuracy, stability, and reliability of measurements, differential navigation (DN) systems have been developed. In a DN system, the position of a user is determined relative to a base station (also referred to as a base) whose coordinates are precisely known. The base contains a navigation receiver that receives satellite signals. The user, whose position is to be determined, can be stationary or mobile and is often referred to as a rover. The rover also contains a navigation receiver that receives satellite signals. Results of base measurements are transmitted to the rover via a communications link. To accommodate a mobile rover, a wireless communications link is used.
The rover processes measurements taken with its own navigation receiver, along with measurements received from the base, to improve the accuracy of determining its position. Many of the errors in calculating coordinates from GNSS measurements, such as satellite clock drifts and signal propagation delays through the ionosphere and troposphere, are highly correlated at the rover and the base, if the rover and the base are sufficiently close.
Since the coordinates of the base are precisely known, errors of the GNSS measurements at the base can be computed by comparing the calculated coordinates to the known coordinates. The errors of base measurements can be used to correct the errors of rover measurements. Usually, a differential global positioning system (DGPS) computes locations based on pseudo-ranges only. With a navigation receiver operating in the DGPS mode, the errors in the rover coordinates are on the order of a meter. Other DN systems, discussed below, provide different accuracies for the coordinates.
In network DN systems, correction information for the rover is generated based on measurements from a group of base stations geographically dispersed over a wide region. A network control center processes the correction information from a particular group of base stations and transmits the correction information to the rover. One example of a network DN system is the commercial OmniSTAR DN system. OmniSTAR provides different grades of service with different accuracies.
Different methods for transmitting the correction information to the rover are used. Radio modems and geosynchronous satellites can be used to re-transmit or re-translate the correction information to the rover. The correction information can be sent over cellular radio channels or over satellite channels.
In the real-time kinematic (RTK) positioning mode, both code and carrier phase measurements at the base station are used for the correction information. The positioning accuracy of RTK systems is on the order of 1 centimeter. Other positioning methods use both code and carrier phase measurements.
U.S. Pat. No. 7,522,099 describes a method for determining the position of a rover relative to an initial location. The method calculates increments of rover coordinates for an epoch by using full phase increments for an epoch. The coordinate increments are added over the time elapsed from the departure time from the initial location. The coordinates relative to an initial location are referred to as local coordinates. Since the local coordinates are generated from coordinate increments that are based on carrier phase measurements, the local coordinates have a higher accuracy than coordinates based on code measurements alone. The determination of local coordinates can be useful for different applications; for example, in surveying locations and distances relative to a monument.
U.S. Pat. Nos. 7,710,316 and 7,439,908 describe the use of carrier phases to smooth coordinates obtained from code measurements. Measured carrier phase increments for an epoch are transformed into rover coordinate increments for an epoch. The coordinate increments and code measurements are then fed into a complex smoothing filter. Integration of the carrier phase and code measurements considerably reduces fluctuations of code coordinates during random motions of the rover, without increasing dynamic errors. Details of methods for measuring carrier phases and algorithms for generating coordinate increments and local coordinates are described in the above-mentioned patents.
To improve accuracy, particular filtering methods can be applied to the measurements. Different filtering methods are described in U.S. Pat. Nos. 7,439,908; 7,193,559; 7,710,316 B1; 7,153,559; 6,664,923; 7,439,908; 6,337,657; and 7,710,316. Filtration is most effective when there are no abnormal (anomalous) errors. U.S. Pat. No. 5,410,750 and U.S. Pat. No. 6,861,979 describe methods for handling anomalous errors. U.S. Pat. Nos. 5,901,183; 6,493,378; 7,212,155; and 6,397,147 describe methods for reducing specific types of errors.
When normal DN operation is disrupted, navigation receivers can switch to a different operational mode with lower accuracy. If a navigation receiver operating in the DGPS mode loses communication with the base station, for example, the navigation receiver can switch over to the stand-alone mode. Similarly, if the normal operation of a navigation receiver in a network DN system is disrupted, the navigation receiver, depending on the specific disruption, can switch over to a lower accuracy grade of service or to the stand-alone mode. Methods and apparatus for improving the positioning quality of navigation receivers when normal DN operation is disrupted are desirable.